1. Field of the Invention
The present invention relates to a semiconductor laser used in an optical disk device such as a CD-R/RW drive, DVD-RAM drive, MD drive or the like.
2. Description of Related Art
In an optical recording device, it has been intended to improve the recording speed. For example, a CD-R drive having a so-called 12-time recording speed is put into practical use. In such an optical recording device increased in recording speed, it is required to invoke a high-output laser light in an instant. As laser which can satisfy such requirements, there is known a ridge-type semiconductor laser using a compound semiconductor.
FIG. 1A to FIG. 1E are schematic section views illustrating the steps of producing a ridge-type semiconductor laser using a compound semiconductor material.
Successively formed on a substrate 1 are a lower clad layer 2, an active layer 3, an upper first clad layer 4, an etching stop layer 5, an upper second clad layer 6, and a contact layer 7. Further formed on the contact layer 7 is a mask layer 8, on which a resist pattern 9 is then formed (FIG. 1A). Each of the semiconductor portions is made of a GaAs-type material, and the mask layer 8 is generally made of SiO2 or SiN.
Then, the mask layer 8 is etched according to the pattern of the resist 9 (FIG. 1B). After the resist 9 is removed, the contact layer 7 and the upper second clad layer 6 are etched in the form of a ridge. In the following description, both the upper second clad layer 6 and the contact layer 7 etched in the form of a ridge, are called a ridge section 12. The etching stop layer 5 is resistant to the etching medium. This prevents the etchings top layer 5 and the layers thereunder from being etched (FIG. 1C).
Then, light confining layers 10 are grown selectively at both sides of the ridge section 12 (FIG. 1D). At this time, no light confining layer 10 should be formed on the mask layer 8. When the light confining layers 10 are made of GaAs, triethylgallium is used as a gallium feeding raw material and a MOCVD (Metal-Organic Chemical Vapor Deposition) method is used, thus causing the light confining layers 10 to be selectively grown.
After the mask layer 8 is removed, a cap layer 11 is grown (FIG. 1E).
In the semiconductor laser having the arrangement above-mentioned, to confine the laser light in the vicinity of the active layer 3, each of the refractive index of the lower clad layer 2 and the refractive index of the upper first clad layer 4 must be lower than the refractive index of the active layer 3.
GaAs absorbs light. Accordingly, when the light confining layers 10 are made of GaAs, it is required to increase the thickness of the upper first clad layer 4 to prevent the GaAs of the light confining layers 10 from absorbing light. This increases the laser operating electric current, resulting in failure in acquirement of a high-output laser light.
In view of the foregoing, the inventor of this application got the idea that the light confining layers 10 are made of an AlGaAs-type material less in light absorption to enable the upper first clad layer 4 to be thinner to increase the oscillation efficiency, thus obtaining a high-output laser light. Then, he tried to form the light confining layers 10 having a composition of Aly2Ga(1-y2)As with the use of triethylgallium as the gallium feeding raw material. As a matter of fact, it was found that the component material of the light confining layers 10 deposited also on the mask layer 8, and that the light confining layers 10 could not selectively be grown in the desired manner. It is noted that such deposits of the component material of the light confining layers 10 will be an obstacle to removal of the mask layer 8.
This problem may be solved by supplying a corrosive chlorine gas or the like at the time when the light confining layers 10 are selectively grown. However, this causes a variety of another trouble relating to handling of corrosive gas.
Further, to increase the light confining effect to prevent the laser light from spreading transversely in the vicinity of the active layer 3, the light confining layers 10 are preferably made of Aly2Ga(1-y2)As of which refractive index is lower than that of the upper second clad layer 6. In an AlGaAs-type crystal, its refractive index is lower as the Al concentration is higher. Accordingly, the Al concentration of the light confining layers 10 is preferably higher than that of the upper second clad layer 6. When there is used an AlGaAs-type semiconductor, the upper second clad layer 6 generally has a composition of Alx3Ga(1-x3)As (0.3 less than x3 less than 0.7). Accordingly, the light confining layers 10 preferably have a composition of Aly2Ga(1-y2)As(0.4 less than y2 less than 1.0)
It is an object of the present invention to provide a semiconductor laser producing method capable of selectively growing, in the desired manner, light confining layers having an AlGaAs-type composition.
In other words, the specific object of the present invention is to provide a highly efficient semiconductor laser producing method.
A method of the present invention comprises the steps of: successively laminating, on a compound semiconductor substrate, a lower clad layer, an active layer, and an upper first clad layer; forming, on the upper first clad layer, an upper second clad layer in the form of a ridge; and selectively growing, at each side of the ridge-shape upper second clad layer, a light confining layer with the use of a III-group element feeding raw material comprising a III-group element compound having a methyl group (preferably without use of a III-group element compound having an ethyl group).
For example, when there is used a III-group element feeding raw material comprising a III-group element compound having a methyl group for forming the light confining layers according to an MOCVD method or the like, the light confining layers can successfully be grown selectively at the sides of the ridge-shape upper second clad layer. Further, according to the method of the present invention, it is not required to use corrosive chlorine gas when forming the light confining layers.
Preferably, the light confining layers have a composition of Aly2Ga(1-y2)As (0 less than y2 less than 1.0).
When the light confining layers are composed of Aly2Ga(1-y2)As (0 less than y2 less than 1.0) less in light absorption, the upper first clad layer can be reduced in thickness, thus enabling the semiconductor laser to supply a high-output laser light. For selectively growing, at the sides of the ridge-shape upper second clad layer, the light confining layers having the composition range above-mentioned, it is effective to use a III-group element feeding raw material comprising a III-group element compound having a methyl group.
More preferably, the light confining layers have a composition of Aly2Ga(1-y2)As (0.4 less than y2 less than 1.0).
To prevent the laser light in the vicinity of the active layer from spreading transversely such that the laser light is confined in the area under the ridge-shape upper second clad layer, the refractive index of the light confining layers must be lower than that of the upper second clad layer. In an AlGaAs-type crystal, its refractive index is lower as the Al concentration is higher. In an AlGaAs-type semiconductor laser, the upper second clad layer generally has a composition of Alx3Ga(1-x3)As (0.3 less than x3 less than 0.7). Accordingly, the light confining layers preferably have a composition of Aly2Ga(1-y2)As (0.4 less than y2 less than 1.0). According to the method of the present invention, the light confining layers having such a composition can be formed.
An embodiment of the method of the present invention comprises the steps of: forming, on a GaAs substrate, a lower clad layer of Alx1Ga(1-x1)As; forming an active layer on the lower clad layer; forming, on the active layer, an upper first clad layer of Alx2Ga(1-x2)As; forming, on the upper first clad layer, a ridge-shape upper second clad layer of Alx3Ga(1-x3)As; and selectively growing, at each side of the upper second clad layer, a light confining layer with the use of a III-group element feeding raw material comprising a III-group element compound having a methyl group, this selective growth being conducted according to an MOCVD method for example.
When the specific component materials of a semiconductor laser are set as above-mentioned, an AlGaAs-type semiconductor laser can be formed. The active layer maybe made of a single layer of Aly1Ga(1-y1)As or multiple layers having two types of different composition layers. More specifically, the active layer may be an MQW (Multi Quantum Well) active layer composed of Aly11Ga(1-y11)As and Aly12Ga(1-y12)As (y11xe2x89xa0y12), or may be an MQW active layer composed of Aly1Ga(1-y1)As and GaAs.
By using a III-group element feeding raw material comprising a III-group element compound having a methyl group, the light confining layers can successfully be grown selectively at the sides of the ridge-type upper second clad layer.
Preferably, the step of selectively growing the light confining layers comprises a step of forming a mask of SiO2 or SiN on the upper second clad layer.
The upper second clad layer may be formed in the following manner. There is formed, on the upper first clad layer, a continuous layer made of the material of the upper second clad layer, and the unnecessary portions of this continuous layer are etched such that the upper second clad layer is made in the form of a ridge. In this case, the mask layer is preferably formed on the upper second clad layer in a continuous form.
By forming the light confining layers with the use of a III-group element feeding raw material comprising a III-group element compound having a methyl group, it is possible to effectively restrain the component material of the light confining layers from depositing on the mask of SiO2 and SiN. More specifically, the light confining layers can successfully be grown selectively at the sides of the ridge-shape upper second clad layer.
Preferably, the III-group element feeding raw material comprises trimethylgallium. In this case, gallium can be fed as a III-group element at the time when forming an AlGaAs-type semiconductor laser.
Preferably, the III-group element feeding raw material comprises trimethylaluminium. In this case, aluminium can be fed as a III-group element at the time when forming an AlGaAs-type semiconductor laser.
Preferably, the selective growth of the light confining layers is conducted at temperature in the range from 500xc2x0 C. to 750xc2x0 C. When the temperature is lower than 500xc2x0 C., the film quality of the light confining layers might possibly be deteriorated. When the temperature exceeds 750xc2x0 C., this involves the likelihood that a film of the component material of the light confining layers is undesirably grown also on the mask. In this point of view, the temperature is more preferably in the range of 600 to 700xc2x0 C.
As thus discussed, according to the method of the present invention, the light confining layers having an AlGaAs-type composition can be grown selectively at both sides of the ridge-shape upper second clad layer. Thus, the resultant semiconductor laser enables the laser light to be confined in the area under the ridge section without the laser light remarkably attenuated. Such a semiconductor laser can oscillate at high efficiency. More specifically, such a semiconductor laser can provide a higher-output laser light at a predetermined electric power.
These and other features, objects and advantages of the present invention will be more fully apparent from the following detailed description set forth below when taken in conjunction with the accompanying drawings.